Adhesion at partially restructured polymer surfaces

Abstract

The controlled modification of the surface structure of styrene-4-vinyl pyridene (SVP) and of ethylene-acrylic acid (EAA) copolymers was achieved by the timed exposure of the polymers to non-orienting (n-alkane) and orienting (water) fluids. The degree of surface modification was monitored by periodic measurements of non-dispersive surface energies from static contact angle data. Assemblies of rigid PVC/SVP, of poly(methyl methacrylate) (PMMA)/EAA and of polypropylene (PP)/EAA were prepared with the SVP and EAA in various stages of restructuring. Bond characteristics of these assemblies were evaluated from single lap-shear measurements. Bond strengths responded strongly to the orientational states of the copolymer adhesives. In PVC/SVP and PMMA/EAA, significant increases in bond strength were observed when orientational changes raised the nondispersive contribution, γ nd, to the surface energies of these copolymers. Surprisingly, full increments in bond strengths were realized at only about 25–40% of the total increment in γ nd. Surface restructuring had little effect on adhesion with the non-polar PP substrate. Residual bond strengths, following controlled aging, varied directly with the initial bond strength. The residual bond strengths may be used to specify ideal restructuring events in the adhesives which would render the assemblies fully resistant to the chosen aging process. Acid/base interaction concepts have been used to rationalize the observations. These suggest that only relatively few polar elements need to migrate into the surface region of the adhesives to maximize initial bond strength, but larger numbers are needed to ensure adequate resistance to aging.